When laying pipelines on the seabed, it is common according to prior art for the lay vessel to lower the free end portion of the pipe down to the seabed.
During pipe maintenance, pigs are sent through the pipe e.g. to clean, check and/or test the pipe. According to prior art, a separate manifold/structure is installed on the seabed by means of a lifting vessel brought to the site, whereupon the pipeline is pulled into and connected to the manifold.
Alternatively, the end portion of the pipeline may be lifted to the surface, where a manifold is welded on. Then the pipeline can be re-installed.
According to prior art, a shroud is normally constructed underneath valves and couplings that are to be installed directly in line with the pipeline. The shroud ensures an even transition between the pipe and the bottom profiles of the valves/couplings, which is required in order to enable the valves/couplings to be passed across rollers on the lay ramp of the lay vessel. Furthermore, the shroud is commonly used to ensure that the end portion of the pipe does not sink into the seabed, whereby the end opening of the pipe would be inaccessible. The underlying shroud will distribute the weight of the pipe and the manifold over a somewhat larger area.
Separate structures of this type require extensive construction and fabrication work. A lack of standardisation makes it necessary to do the construction work again for each pipe, and to tailor e.g. the maintenance work to each particular case. The underlying shroud at the end portion of the pipe must extend along a relatively long section of the end portion of the pipe in order to be able to pass across the lay ramp of the lay vessel. Valves welded directly to the pipeline are subjected to relatively large bending moments, thus being prone to damage or becoming less reliable in operation.
It appears that the risk of hydrocarbon leaks/spills increases with the number of connections. Thus the relatively large number of connections associated with prior art manifolds is a disadvantage. Installing a pipe with a separate manifold involves the lay vessel having to spend extra time in order to arrange and connect the manifold to the pipe, or optionally the use of a lifting vessel to carry out the connecting operation. Consequently, the connecting operation is unnecessarily expensive.